1. Field of the Invention
The invention relates to an organic electroluminescent display device having a thin film transistor in each of pixels.
2. Description of the Related Art
An organic electroluminescent (referred to as “EL”, hereafter) display device using an organic EL element as a self-emissive element has been developed as a display device replacing a CRT or an LCD in recent years. Particularly, an active matrix type organic EL display device that has a thin film transistor (referred to as “TFT”, hereafter) driving an organic EL element in response to a video signal in each of the pixels has been developed.
The TFT is formed on a glass substrate. Therefore, in a bottom emission type organic EL display device where light from the organic EL element is emitted through this glass substrate, external light enters an active layer of the TFT through the glass substrate. This external light excites carriers in the active layer if it is strong, and a photocurrent (off-state leak current) flows between a source and a drain to cause a crosstalk or the like, thereby degrading display contrast.
A technology where a photocurrent is prevented by providing a light shield film shielding an active layer of TFT from external light is known as described in Japanese Patent Application Publication No. 2004-134356.
However, when the light shield film is made of a conductive material, there is such a problem caused by a back channel that a variation in characteristics (e.g. a threshold value) of the TFTs or a display failure by a short circuit between the light shield film and the active layer occurs.